The quest Land Owner Needs Manual Laborers says slavery of bandits for manual labor is accepted in Calradia. The player should be able to exploit this themselves. Simply make it so that every prisoner (or just bandit type prisoners) in your dungeon gives a boost to construction speed, either cumulative with or as an alternative to paying money. There's also currently no real reason to keep nameless prisoners in the dungeon besides stockpiling them (since roguery XP seems to get a bonus from selling in bulk), so this would give an incentive to do so. For balance, perhaps have the dungeon population slowly decrease while doing this, to simulate forced laborers dying off from harsh conditions and having some escape chance?